Opto-electronic frequency divider circuit and method of operating same

ABSTRACT

The circuit includes an electro-optical mixer, such as an electro-optical Mach-Zehnder modulator ( 2 ), with non-linear behaviour. The modulator ( 2 ) receives as input an optical signal (P in ) at the frequency to be divided, in addition to an electric signal (e 3 ) at a given frequency, usually corresponding to the frequency deriving from such division. The output optical signal (P out ) from modulator ( 2 ) exhibits a modulation spectrum containing the frequency difference between the frequency to be divided and at least one harmonic of the frequency of the above electric signal. After having been converted into an electric signal (e 1 ), the output signal of the mixer is subjected to a filtering action ( 8 ) to extract the above frequency difference component. This latter one is then used both as electrical signal (e 3 ) for the mixing, and as output signal from the divider (e 2 ). The preferred application is to OTDM systems, to extract a synchronism signal at tributary signal frequency.

FIELD OF THE INVENTION

The present invention refers to frequency divider circuits and has beendeveloped by paying particular attention to their possible applicationto optical transmissions based on the so called OTDM (Optical TimeDivision Multiplexing) technique.

BACKGROUND OF THE INVENTION

OTDM is particularly interesting in all situations where the need arisesto increase the transmission capacity of an optical link, and it is analternative to other solutions based on sharing the same physicalcarrier among multiple channels—typically by employing WDM (WavelengthDivision Multiplexing) techniques—or on increasing the number of opticalfibers available for the link.

This latter solution generally requires interventions on theinstallation (such as laying of new cables and performing excavations)and it does not completely exploit the extremely wide band madeavailable by optical fibers.

The simultaneous transmission of many different channels on the sameoptical fiber, for example according to wavelength division multiplexingtechniques, allows use of low speed opto-electronic components both inthe transmitter and in the receiver, while obtaining a high aggregatecapacity on the link. Wavelength division multiplexing further allowsimplementing at optical level some network functions like channelremoval and insertion, dynamic routing, link protection, with a highreduction of the processing load for the electronic part in networknodes. The major inconveniences of such method are linked to the need ofselecting and stabilizing the wavelengths for transmitters and opticalfilters used for channel selection, to the possible inter-channelinterference due to non-linear phenomena in fiber propagation (forexample the phenomenon known as Four Wave Mixing) or to the spectralnonuniformity of optical amplifier gain.

In OTDM systems, many optical signals, intensity modulated according toan RZ (return to zero) code, are interleaved into a single flow byacting on the relative delay of the pulse sequences. This solutionretains the majority of advantages of WDM techniques related to thepossible use of low speed opto-electronic components both in thetransmitter and in the receiver, further avoiding the onset of some ofthe above-mentioned negative phenomena. A basic condition for the properoperation of an OTDM system is however that the different opticaltributary flows must be well synchronized and composed of sufficientlynarrow pulses in order to avoid interference among channels. Moreover,it is essential that a driving signal at tributary frequency andsynchronous with the multiplexed flow be made available at thedemultiplexing device.

OBJECT OF THE INVENTION

The object of the present invention is to provide a solution to thislatter need and, more generally, to provide a particularly simpleopto-electronic frequency divider circuit, adapted to operate even atvery high frequencies (typically with input frequencies of the order ofseveral tens of Gbit/s) with good performance as regards the stabilityof frequency and phase locking between the signal resulting from thedivision and the input signal.

SUMMARY OF THE INVENTION

According to the present invention, such object is attained by means ofan opto-electronic frequency divider circuit.

Opto-electronic frequency divider circuit which comprises:

electro-optical mixer means with a nonlinear behavior, adapted toreceive as input a first optical signal (P_(in)) at a frequency to bedivided (f₀) in addition to an electric signal (e₃) at a given frequency(f₁) and to generate as output a second optical signal (P_(out)) whosemodulation spectrum includes, due to the mixing action, the frequencycorresponding to the difference between the frequency to be divided (f₀)and at least one harmonic of the given frequency (f₁) generated due tothe nonlinear behavior,

a feedback path comprising opto-electronic converter means to convertthe second optical signal (P_(out)) into an electrical conversion signal(e₁) adapted to be sent back to the electro-optical mixer means,

filtering means associated with the feedback path to extract from thespectrum the component at the difference frequency, and

extracting means to derive from the feedback path as signal (e₂)resulting from the frequency division action, a signal at the differencefrequency.

The electro-optical mixer means can include a Mach-Zehnderelectro-optical modulator.

The electro-optical mixer means can be provided with a control input(V_(bias)) to select the order of the at least one harmonic. Theelectro-optical mixer means can exhibit a substantially sinusoidaltransmittivity/input voltage characteristics, and in that theelectro-optical mixer means are made to operate next to one of theintermediate points in the characteristics, so that the at least oneharmonic is an odd-order harmonic. This harmonic can be the thirdharmonic of the given frequency.

The feedback path can include at least one delay element which may be anoptical waveguide interposed between the electro-optical mixer means andthe opto-electronic converter means. Alternatively the delay element canbe a delay line operating on electrical signals and located, along thefeedback path, downstream of the opto-electronic converter means. Withinthis feedback path, the filtering means are located downstream of theopto-electronic converter means. The feedback path can include gaincontrol means to keep the electrical signal (e₃) at such a level as toensure the nonlinear behavior of the electro-optical mixer means.

The filtering means can be connected in the feedback path, so that thecomponent at the difference frequency is used as the electric signal(e₃) fed to the electro-optical mixer means.

The extracting means can be located downstream of the filtering means sothat the component at the difference frequency is used as signal (e₂)resulting from the frequency division action.

The first optical signal (P_(in)) can be a signal belonging to anaggregate flow obtained by optically time division multiplexing aplurality of tributary flows, each one having a bit rate equal to thegiven frequency (f₁), and the signal (e₂) resulting from the frequencydivision action can be a signal synchronous with the tributary flowsthat forms a synchronism signal for demultiplexing the aggregate flow.

The invention also comprises a method of extracting from an opticalsignal (P_(in)) conveying an aggregate flow of a given number (N) ofoptical tributary signals interleaved according to an optical timedivision multiplexing scheme, a synchronism signal at the frequency (f₁)of the optical tributary signal. The method includes the followingoperations:

subjecting the optical signal (P_(in)) to a nonlinear electro-opticalmixing operation with an electric signal (e₃) at the frequency (f₁) ofthe optical tributary signals in order to generate a further opticalsignal (P_(out)) having a modulation spectrum including, due to themixing operation, a frequency corresponding to the difference betweenthe frequency of the aggregate flow (f₀) and at least one harmonic ofthe frequency (f₁) of the optical tributary signals, generated due tothe nonlinear behavior of the mixing operation,

converting the further optical signal (P_(out)) into an electricalconversion signal (e₁) that can be used to generate the electricalsignal (e₃) for the electro-optical mixing operation, according to ageneral feedback path to which a filtering operation is associated toextract from the spectrum the component at the difference frequency, thesignal thereby extracted being the synchronism signal.

The method, when applied to an aggregate flow of N optical interleavedtributary signals is characterized in that the electro-opticalconversion operation is performed with such a nonlinearity degree thatthe at least one harmonic is an (N−1)th-order harmonic. The method caninclude the operation of delaying propagation of at least one of thesteps of:

(1) delaying propagation of the further optical signal (P_(out)) withinthe feedback path,

(2) delaying propagation of the electrical conversion signal (e₁) withinthe feedback path, and

(3) delaying propagation of both the further optical signal (P_(out))and the electrical conversion signal (e₁) within the feedback path.

In the preferred but not exclusive application to an OTDM transmissionsystem, this circuit allows performing a division of the aggregate bitrate by employing components with a bandwidth not exceeding thetributary bit rate. The practical embodiment of the frequency dividertherefore does not require a technological development degree higherthan the one necessary to make the transmitter and receiver of thesingle channel in the transmission system.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 shows, in block diagram form, the general arrangement of an OTDMoptical transmission system to which a frequency divider circuitaccording to the invention can be applied;

FIG. 2 shows—also in block diagram form—the general structure of thedivider circuit according to the invention; and

FIG. 3 is a diagram showing an operating characteristic of one of thecomponents of the circuit in FIG. 2.

SPECIFIC DESCRIPTION

In FIG. 1 reference T globally designates an optical link realisedaccording to the OTDM technique.

System T, operating according to criteria known per se, allows conveyingon an optical fiber line W (with associated respectiveamplification/equalisation units schematically represented by blocks AE1and AE2) an RZ optical signal intensity modulated according to RZ(return to zero) code and having a bit rate f₀ equal, for example, to 40Gbit/s.

Such optical signal is obtained by aggregating in a multiplexing unit(optical coupler) OC a number of optical signals (four in the example),defined as “tributary”, each one with bit rate f₁ equal to 10 Gbit/s.

It is clearly apparent that reference to an aggregate flow with bit rateequal to 40 Gbit/s, obtained through multiplexing four 10 Gbit/stributary flows, must be deemed purely as an example. Both the bit ratefor tributary signals, and the number of such multiplexed signals,and—consequently—the bit rate of the resulting aggregate flow are designparameters adapted to be widely modified as a function of the specificapplication needs and of the components being used, without departingfrom the scope of the present invention.

In particular, the tributary flows (hereinbelow in the presentspecification four tributary flows will always be referred to, as anexample) are generated starting from the RZ pulse train issued by apulse source S, typically composed of a laser source with sufficientlynarrow pulses that are time-domain limited. The signal from source S issplit by a separator SP1 into a plurality of replicas each fed to arespective modulator M1 to M4.

The transmitter input data, in the form of electric signals ideallycoming from a data source SD synchronously driven with source S due tothe common slaving to a synchronism oscillator SYN (also operating, inthe disclosed embodiment, at a frequency of 10 GHz, equal to thefrequency of source S), are organised in a corresponding number ofchannels C1-C4, each with a bit rate of 10 Gbit/s. The signals presenton channels C1-C4 drive the respective optical modulators M1-M4. Thelatter ones “write” the information on the related pulse trains and thesignals obtained are sent to fiber W through optical coupler OC afterhaving been mutually time-offset by a time interval Δt, for example inrespective adjustable optical delay lines L1-L4.

The aggregate flow injected into fiber W thereby shows a typicaltime-domain multiplexed structure. In practice, the aggregate flowconveyed by fiber W is cyclically composed of a symbol corresponding toa datum coming from channel C1, a symbol corresponding to a datum comingfrom channel C2, a symbol corresponding to a datum coming from channelC3, a symbol corresponding to a datum coming from channel C4, etc.

At the receiving side, the aggregate flow is sent to an opticaldemultiplexer DMPX that orderly extracts from the received aggregateflow the signals corresponding to the tributary channels and routes themtowards corresponding receivers RX1-RX4 to recover at the output theflows corresponding to channels C1-C4.

To be able to correctly extract the different tributary signals, thedemultiplexer device DMPX requires a driving signal e₂ (which issupplied by a synchronism recovery circuit globally designated as 1 andwhich is to be also sent to receivers RX1-RX4) at the tributaryfrequency and phase locked with the aggregate (or multiplexed) flowfrequency.

When operating in RZ format, a line at the aggregate flow bit rate isalways present in the multiplexed flow spectrum, while the spectralcomponent at tributary frequency is absent (it disappears due to themultiplexing operation). In order to correctly operate, the synchronismrecovery circuit must perform a division of the aggregate bit rate by afactor equal to the multiplexing factor, while keeping the phase lockingbetween the original frequency and the divided one.

In the specific case, the signal corresponding to aggregate bit rate isextracted from the signal coming from fiber W through a component likeseparator SP2.

Synchronism recovery circuit 1, that is the subject matter of thepresent invention, is therefore a circuit that, starting from theoptical signal at the aggregate frequency, is able to generate anelectric signal with a frequency corresponding to the tributary signalfrequency.

The circuit according to the invention ideally refers to a circuit knownas Miller frequency divider. Such divider circuit, also known asregenerative divider since it is composed of a feedback system, ischaracterised by very low added noise levels. It found thereby use ingenerating sources with high spectral purity, obtained by dividing highfrequency references. For a general description of the features of suchknown circuit, reference can be made to the paper by R. C. Miller“Fractional-frequency Generators Utilizing Regenerative Modulation” inProceedings. IRE, Vol. 27, pages 446-457, July 1939.

Similarly to the Miller divider, circuit 1 according to the inventionuses a component adapted to perform a mixer function. Here amultiplication is performed between the component at the frequencycorresponding to the pulse repetition rate of the signal to be divided,and the harmonics of the electrical division signal coming from thefeedback loop. Such harmonics originate inside such component,designated with reference 2 and characterised by a non-linear behaviour.

Any component showing this type of features can then be used in theinvention. In the currently preferred embodiment, component 2 is anelectro-optical Mach-Zehnder modulator having transmittivitycharacteristics as shown in the diagram in FIG. 3.

In such a diagram, the ordinate axis shows transmittivity T (normalizedto unity as maximum value) versus a parameter that can be expressed as(V_(bias)+V_(RF))/V_(π), and that is characteristic of the drivingconditions for modulator 2.

In particular, V_(bias) represents a bias voltage applied to a firstdriving input of modulator 2, while V_(RF) is a radiofrequency drivingsignal applied to a corresponding input. In the specific embodimentshown in FIG. 2, the concerned radiofrequency signal is designated ase₃. Parameter V_(π) is a normalization parameter: in practice, it is thevoltage to be applied to move from maximum to minimum transmittivity.Obviously the transmittivity refers to the input/output behavior of theoptical signal passing through the modulator. In the example shown, theoptical input signal is represented by signal P_(in) while the outputsignal, designated as P_(out), is equal to the input signal multipliedby transmittivity. With modulator 2 it is therefore possible to performa multiplication between the spectral components in optical input powermodulation and the driving signal harmonics.

If the radiofrequency signal V_(RF) is a sufficiently strong sinusoidalsignal, transmittivity includes components with frequenciescorresponding to the frequencies of the harmonics of signal e₃. Therelative amplitudes of such harmonics depend on the working pointposition on the modulator characteristics (see FIG. 3), and therefore onbias voltage V_(bias). In particular, it is possible to have evenharmonics only, by placing the working point next to maximum or minimumtransmittivity, or only the odd ones, by operating at thecharacteristics centre.

In the specific embodiment shown here, it is desired to perform afrequency division by four (N=4). Therefore, the choice has beenoperating at the characteristics center, that is next to one of theinflexion points shown by arrows in FIG. 3, in order to generate and usethe third harmonic (N−1=3) of the radiofrequency signal.

In other words, the following results are obtained:

generation (due to non-linear behavior), starting from signal e₃ atfrequency f₁, of the (N−1)th order harmonic at frequency (N−1)f₁, and

generation (due to the typical multiplying behavior of modulator 2) ofan output signal whose frequency is equal to the difference between theinput signal frequencies (in addition to a signal with sum frequency).

In general, since frequency f₀ conveyed by input signal P_(in) is Ntimes frequency f₁ (f₀=Nf₁), the output difference signal will have thefollowing frequency

Nf ₁−(N−1)f ₁ =f ₁

this latter one being the desired frequency.

Obviously, by choosing a different working point and/or a component withdifferent non-linear characteristics, it is possible to generate adifferent-order harmonic and therefore, to perform a division by adifferent factor. In any case it is important to note that, sinceharmonic generation occurs inside modulator 2, it is not necessary thatthe passband of radiofrequency input of modulator 2 itself extendsbeyond the frequency of signal with frequency f₁.

In summary, in the diagram in FIG. 2, the optical input signal P_(in),intensity modulated at the frequency f₀ to be divided (40 Gbit/s in thementioned example) originates, through electro-optical modulator 2,optical signal P_(out). This latter one is received, possibly through asection of optical fiber 3 operating as delay line, by a photodiode 4.In photodiode 4 the optical signal P_(out) is converted into an electricsignal e₁. After passing through a variable electric delay line 5located downstream of photodiode 4, an amplifying stage 6 and a variableattenuator 7 (whose function will be better described below), signal e₁is filtered in a pass-band filter 8 tuned around frequency f₁. Thefiltered signal thus obtained, designated as e₃, is sent back to inputV_(RF) of modulator 2, thus providing a feedback. Usually, when passingfrom filter 8 (also located downstream of photodiode 4 within thefeedback path) to modulator 2, the signal is also made to pass throughan amplitude-adjusting amplifying stage 9 and an extracting device 10,such as for example a directional coupler. The latter one extracts afraction of the feedback signal intended to build the output signal e₂for the device.

The circuit performs a phase locking between input signal P_(in) andoscillating signal e₃ and this makes the circuit itself useful forapplications like the tributary synchronism recovery device in thediagram in FIG. 1.

Experiments carried out by the Applicant have demonstrated theadvantages of employing, within the general diagram in FIG. 2, someparticular application embodiments. These embodiments are to beconsidered generally preferred, even if not mandatory per se.

In particular, the input signal P_(in) is preferably injected intomodulator 2 through a polarisation controlling member, of a known typeand not specifically shown.

The use of optical fiber 3, responsible for the majority of delaysincurred by signals in the feedback path, is generally deemed preferredin order to stabilise the circuit operation, when necessary. Thevariable delay line 5 provided in the electrical section of the feedbackloop is necessary for the fine control of the global phase shift.Preamplifier 6 and variable attenuator 7 allow accurately adjusting thecirculating power.

The pass-band filter 8 does not require particular selectivityproperties. Amplifier 9 is to raise signal e₃ supplied to input V_(RF)of modulator 2 to a sufficiently high level (for example about 25 dBm)so that such signal, inside modulator 2, is subjected to the necessarydistortion to produce the desired harmonic.

Obviously, without changing the principle of the invention, componentparts and embodiments can be largely modified with respect to what isdescribed and shown, without departing from the scope of the presentinvention.

What is claimed is:
 1. A method of extracting from an optical signal(P_(in)) conveying an aggregate flow of a given number (N) of opticaltributary signals interleaved according to an optical time divisionmultiplexing scheme, a synchronism signal at the frequency (f₁) of saidoptical tributary signals, said method comprising the steps of:subjecting said optical signal (P_(in)) to a nonlinear electro-opticalmixing operation with an electric signal (e₃) at the frequency (f₁) ofsaid optical tributary signals in order to generate a further opticalsignal (P_(out)) having a modulation spectrum including, due to themixing operation, a frequency corresponding to the difference betweenthe frequency of said aggregate flow (f₀) and at least one harmonic ofthe frequency (f₁) of said optical tributary signals, generated due tothe nonlinear behavior of said mixing operation; converting said furtheroptical signal (P_(out)) into an electrical conversion signal (e₁) thatcan be used to generate said electrical signal (e₃) for theelectro-optical mixing operation, according to a general feedback pathto which a filtering operation is associated to extract from saidspectrum the component at said difference frequency; extracting, fromsaid feedback path a signal (e₂) corresponding to said differencefrequency, the signal thereby extracted being said synchronism signal,said electro-optical mixing operation is performed with such anonlinearity degree that said at least one harmonic is an (N−1) orderharmonic; and delaying propagation by at least one of the steps of (1)delaying propagation of the further optical signal (P_(out)) within thefeedback path, (2) delaying propagation of the electrical conversionsignal (e₁) within the feedback path, and (3) delaying propagation ofboth the further optical signal (P_(out)) and the electrical conversionsignal (e₁) within the feedback path.
 2. The method defined in claim 1wherein said mixing is carried out in a Mach-Zehnder electro-opticalmodulator.
 3. The method defined in claim 2 wherein said mixing iscarried out by providing a control input (V_(bias)) to select the orderof said at least one harmonic.
 4. The method defined in claim 3 whereinsaid at least one harmonic is an odd-order harmonic.
 5. The methoddefined in claim 4 wherein said at least one harmonic is a thirdharmonic of the frequency (f₁).